An MIT-Harvard Project: The Electron Accelerator
Up on Oxford Street, beyond Mallinckrodt and the University Museum, construction is proceeding on a unique project that will probably prove one of the most valuable experimental devices yet conceived for physical research.
The Cambridge Electron Accelerator, a joint undertaking of MIT and Harvard, was begun in April 1956 and is now about half completed. A two-story administration and laboratory building has been in use for the past six months, and the huge circular tunnel that comprises the accelerator itself is nearly ready. Still to come are a powerhouse and an experimental building which will house the various instruments of measurement.
Since the accelerator is a unique machine, says M. Stanley Livingston, professor of physics at MIT and director of the project, the design of both the tunnel and the measuring apparatus requires a good deal of original work. To this end, physicists and engineers from MIT and Harvard have been busy for the past two years. Livingston expects construction to be finished early in 1960, and, after the machine is "turned up" and tested, operation should begin later that year.
When completed, the Cambridge Electron Accelerator will be the highest energy electron machine in the world. Its maximum energy of 6 billion electron volts is approximately five times as great as that of the largest accelerators now in operation--one at Cal Tech and the other at Cornell. The largest accelerator of any kind is the 30 billion electron volt proton accelerator under construction at the Atomic Energy Commission's Brookhaven laboratories on Long Island.
According to Livingston, the electron accelerator is not properly an "accelerator" in the normal sense of the word. An experiment may start with electrons at .99 the speed of light and increase the velocity to .99999 the speed of light--not too great a change in percentage terms. What does increase dramatically is the energy (and the mass). At the beginning of an experiment, a beam of electrons may have a rest energy of 1/2 million electron volts; it is directed through a linear "pre-accelerator" (or "injector") where the energy is increased to 20 million electron volts. Then, in the accelerator proper, the energy may be raised to the maximum of 6 billion electron volts.
Meanwhile, the mass undergoes a similar increase: the rest mass of a proton is 1800 times as great as that of an electron; by the end of an accelerator experiment, says Livingston, an electron may become "over 6 times as heavy as a proton." In other words, the mass of the electron is increased 12,000 fold; thus, Livingston notes, many physicists half-seriously call the machine a "ponderator."
In the accelerator, electrons whirl around on a constant orbit of 236-foot diameter between a series of 48 strong-focusing magnets. The circular tunnel which encloses it will have a powerhouse in the middle to supply the energy for the magnets. The accelerator tunnel and the powerhouse will be connected by four radial tunnels.
Once the electrons have been accelerated or "ponderated," the aim is to direct a beam into the experimental area where measurements can be made of the unusual properties and inter-actions. One method already devised is to place a target in the part of the electron beam; this will transform the electrons into X-rays of very high energy. Because they are uncharged, these rays will not be deflected by the magnetic field of the accelerator and will continue on a straight tangential line into the experimental area. Physicists would also like to be able to deflect a beam of electrons directly into the experimental area, but this method has not yet been perfected, according to Livingston, and planning is now in process.
The purpose of accelerator experiments, Livingston says, is to "learn more about nuclear force,' to find out "why certain particles (protons, electrons, neutrons, etc.) are the only stable forms of matter." He believes that this new and unique electron accelerator will accomplish much toward answering these questions.
Although Livingston is a member of the MIT faculty, the Cambridge Electron Accelerator is administratively a department of Harvard University.
When it is completed, Livingston says, the accelerator will be available for the "research activities of the physics departments of both schools." Presumably physicists from other schools in the Boston area will also have the opportunity to conduct experiments there.
Everything in the new administration and laboratory building indicates the joint nature of the project. There is an equal distribution of chairs bearing the Tech and Harvard insignia. Livingston has on the wall of his office the two-handled shovel with which the President of Harvard University and the Chancellor of MIT broke ground for the project last year.
Harvard's facilities for nuclear research date back quite a while. Before the Second World War, the University owned a small, constant frequency cyclotron ("the only kind available at that time," says William M. Preston, director of the current cyclotron laboratory). During wartime, however, this machine was appropriated by the government and taken out to Los Alamos for use in the experiments that led to the atomic bomb.
After the war, the Harvard physics department felt that it needed a new cyclotron, so it approached the Office of Naval Research, which supports basic scientific research in the universities. The Navy acquiesced, and construction was begun in 1946 at an Oxford Street site directly behind the new Cambridge Electron Accelerator.
The machine was operating by the middle of 1949, accelerating protons to a maximum energy of 90 million electron volts. At that time, it was one of the largest operating cyclotrons in he world. Research was carried on until 1955, when the cyclotron was shut down for overhaul and modification. It was operating again in the fall of 1956, with a new maximum energy of 160 million electron volts.
At present, Preston says, the Harvard machine can be considered an "intermediate energy cyclotron;" there are about half a dozen larger accelerators of this type in the world. Beyond a certain level of energy (about 600 million electron volts), he says, this particular type of machine (utilizing a single large magnet) is no longer practicable, because of the large size of the magnet required. Therefore, new methods involving a series of magnets, such as those used in the Cambridge Electron Accelerator and the Brookhaven cyclotron, had to be devised.
The chief difference between the cyclotron and the Electron Accelerator, according to Preston, aside from the obvious fact that the former works with protons and the latter with electrons, is one of method. In the cyclotron the protons are subjected to a constant magnetic field and spiral out in ever-increasing orbits; in the election accelerator, on the other hand, the orbit is constant and the magnetic field is increased in order to keep the electrons in a stable path. The cyclotron, Preston says, unlike the C.E.A., can properly be called an "accelerator," for the velocity at the end of the experiment is still "only a few tenths of the speed of light."
At the end of the acceleration process in the cyclotron, the beam is directed into an experimental area. There are, however, no serious problems here of deflecting the particles: once the spiraling beam reaches the outer limit of the cyclotron chamber, it can be made to fly off on a tangential line into the experimental area. A typical experiment, Preston says, might involve a beam hitting a target of some element, perhaps carbon. Counters would be set up at various angles to the beam at the point of collision, and the data thus obtained--used with the laws of momentum--might give important information as to the nature of the carbon nucleus.
With Harvard's cyclotron and other laboratories and the imposing facilities of MIT already in operation, and with the unique C.E.A. under construction, the city of Cambridge may well be considered one of the world's great centers for nuclear research.